A used ProHeat unit I acquired had a broken pump impeller. Based on recommendations on this forum and others I replaced it with a 12V March pump. MUCH cheaper than the replacement ProHeat pump. Motor and pump look better built than original pump. Same specs. It is listed as used in hot water applications. Best price I found online was at Tesco Pumps. In my opinion a good replacement in my application. I have no interest/relationship with Tesco or March.

Does your bus have the auxiliary pump in the back for heater water circulation? Very nice looking brass pump. I took mine out, but plan on using it for added circulation in my hydronic heat system. Rebuild parts are listed in the Blue Bird catalog. Perhaps used are available.

I have the scholastic version Webasto heater which is supposed to plumb into the cooling system of the bus but the PO plumbed it as a seperate system with its own pump. I'm sure you could use a march pump or any DC pump with a high enough water flow and a 1" connection. Make sure the pump has enough flow to keep the heater from getting too hot.